After completing law school, Redstone served as special assistant to U.S. Attorney General Tom C. Clark (who later served as Associate Justice of the Supreme Court of the United States from 1949 to 1967)[3] and then worked for the United States Department of Justice Tax Division in Washington, D.C. and San Francisco, and thereafter entered private practice. In 1954, he joined his father's theater chain, National Amusements and in 1967, he became CEO of the company.[3] As the company grew, Redstone came to believe that content would become more important than distribution mechanisms: channels of distribution (in varied forms) would always exist, but content would always be essential (Redstone coined the phrase, "Content is king!"[8]). He invested in Columbia Pictures, Twentieth Century Fox, Orion Pictures, and Paramount Pictures (Redstone's Viacom would buy Paramount in the 1990s), all of which turned over huge profits when he chose to sell their stock in the early 1980s.

In 1979, he suffered severe burns in a fire at the Copley Plaza hotel, in Boston, but survived after thirty hours of extensive surgery at Massachusetts General Hospital. Though he was warned that he might never be able to live a normal life again, eight years later he was fit enough to insist on playing tennis nearly every day and to launch a hostile takeover of Viacom.[9] Redstone has discussed the story of surviving the fire as a reflection of his strong determination and will to live.[10]

Viacom also owned MTV Networks (formerly known as Warner-AMEX Satellite Entertainment), which owned MTV and Nickelodeon. In addition, other properties included Showtime Networks (a pay-television network similar to HBO and Cinemax) and The Movie Channel. Viacom acquired MTV Networks in 1985 for $550 million from Steve Ross' Warner Communications. (WCI bought American Express' share and then sold the entire entity to Viacom, as they felt that they could not make a lot of money from the venture and the bias of a studio owning cable channels would be a conflict of interest. The studio's stance changed in 1995, when as Time Warner it bought Turner Broadcasting.)

Redstone replaced the team of Jonathan Dolgen and Sherry Lansing in 2004 after their nine-year winning streak ended.[17] Along with the strong slate of films they oversaw, Dolgen and Lansing’s accomplishments included: doubling the size of Paramount's music publishing division, Famous Music; expanding UCI Cinemas into 13 foreign countries; creating the Digital Cinema Initiatives standards body for the new digital film technology; and launching the UPN Network (later part of CBS and now called the CW). The current Paramount Pictures consists only of the movie studio, the other groups having been sold or parceled out to other divisions.

Since arriving at Paramount in 2005, Chairman and Chief Executive Officer Brad Grey has led a return to fortune at the box office.[18][19] He has overseen the creation or revitalization of several major franchises, including Transformers, Star Trek and Paranormal Activity.[20] Paramount has also forged productive relationships with top-tier filmmakers and talent including J.J. Abrams,[21] Michael Bay[22] and Martin Scorsese.[23] The 2010 Paramount slate achieved much success with Shutter Island and a True Grit remake, reaching the biggest box office totals in the storied careers of Martin Scorsese and the Coen Brothers, respectively.[24][25] In addition, during Grey’s tenure, Paramount launched its own worldwide releasing arm, Paramount Pictures International, and has released acclaimed films such as An Inconvenient Truth, Up in the Air, and There Will Be Blood.[26]

The Paramount acquisition was only the tip of the iceberg. He purchased Blockbuster Entertainment, which included Aaron Spelling's production company and a huge library of films, much of which has been merged into Paramount Pictures. Blockbuster has now been spun off into its own independent entity. Redstone acquired CBS Corporation in 2000 and then spun it off as a separate company in 2005, taking with it all of Paramount's television shows and catalog. Following the CBS and Blockbuster Spinoffs, Viacom consists of MTV Networks (MTV, Nickelodeon, VH1, Noggin etc.), music publishing (Famous Music) and Paramount Pictures.

In December 2005, Redstone announced that Paramount had agreed to buy DreamWorks SKG for an estimated $1.6 billion. The acquisition was completed on February 1, 2006. A subsequent financing brought Viacom's investment down to $700 million. The animation studio, DreamWorks Animation, was not included in the deal as it has been its own company since late 2004. However, Paramount now has the rights to distribute films by DreamWorks Animation.

On June 1, 2012, Paramount Pictures renamed the Administration Building on the studio lot the Sumner Redstone Building in a dedication ceremony attended by employees of Paramount Pictures and Viacom.

Redstone's trusts made it clear that his daughter, Shari Redstone (Vice-Chairwoman of the Board of Viacom and CBS as well as President of National Amusements), was set to assume his role upon his death. However, a November 22, 2006, New York Times article indicated that Redstone was reconsidering his daughter's role. In 2007, they feuded publicly over issues of corporate governance and the future of the cinema chain.[28]

Documents were made public which verify that, as part of a settlement from Sumner's first divorce, all of Sumner's stock is in irrevocable trusts that will be left for his grandchildren.[29] On March 1, 2010, Sumner publicly confirmed that all of his stock would be left for his five grandchildren (Brandon Korff, Kimberlee Korff, Tyler Korff, Keryn Redstone, and Lauren Redstone).[30]

Redstone made arrangements to step down as CEO of Viacom in 2006. After Mel Karmazin resigned in 2004, two heirs apparent were named: Co-President and Co-COO Leslie Moonves (who was number 2 to Karmazin at CBS; he was the former head of Warner Bros. Television and before that, Lorimar Television) and Co-President and Co-COO Tom Freston (who had been President and CEO of MTV Networks since 1987 and had been with the company since the formation of MTV Networks' precursor company, Warner-AMEX Satellite Entertainment). After the Viacom split was approved by the board on June 14, 2005, Moonves headed CBS, and Freston headed the new Viacom, Inc.

On September 5, 2006, Redstone removed Freston as President and CEO of Viacom and replaced him with director and former Viacom counsel Philippe Dauman. Redstone also brought back former CFO Tom Dooley. This was surprising to many, as Freston had been seen by many as Redstone's heir apparent, and Redstone had touted that Freston would run the company after he retired. Redstone publicly stated that he let Freston go because of Viacom's lack of aggressiveness in the digital/online arena, lack of contact with investors, and a lackluster upfront (coupled with falling viewership) at MTV Networks.[31][32]

In February 2016, at age 92, after a court-ordered examination by a geriatric psychiatrist whose findings were not publicly disclosed, Redstone relinquished the chairmanship of CBS to Moonves and the chairmanship of Viacom to Dauman.[33][34][35] In May 2016, Los Angeles Superior Court Judge David Cowan dismissed a lawsuit alleging that Redstone was mentally incompetent,[36] although the judge stated it was "not in dispute that Redstone suffers from either mild or moderate dementia".[37] Two weeks later, another such lawsuit was filed in Massachusetts.[38]

Currently, Redstone owns over seventy percent of the voting interest of Viacom. Viacom and CBS Corporation are both controlled by Redstone through National Amusements. Redstone sold his holdings of Midway Games, of over 89 percent, in December 2008.[39]

Redstone's autobiography, A Passion to Win[40] (co-written with author Peter Knobler), was published in 2001 by Viacom's Simon & Schuster. This book details everything from Redstone's life as a young boy in Boston to the difficult takeover of Viacom and the problems he overcame in purchasing and managing both Blockbuster Video and Paramount Pictures. The book also recounts the legendary CBS merger (Viacom was a spin-off company of CBS to syndicate its programs, and the subsidiary bought the parent almost 30 years later).

Viacom's broadcasting properties at the time of A Passion to Win's release included several radio stations and two TV stations: WBZ CBS 4, which had just become a CBS O&O through a merger with Westinghouse four years before Viacom and CBS merged, and WSBK UPN 38 in Redstone's hometown, Boston.

Sumner Redstone has contributed over $150 million to various philanthropic causes.[44]

In April 2007, Sumner M. Redstone announced a commitment of $105 million in charitable grants to fund research and patient care advancements in cancer and burn recovery at three major non-profit healthcare organizations. The cash contributions of $35 million each will be paid out over five years to FasterCures/The Center for Accelerating Medical Solutions, based in Washington, D.C.; the Cedars-Sinai Prostate Cancer Center in Los Angeles, California; and the Massachusetts General Hospital in Boston, Massachusetts.[45]

Over the last few years, Redstone has contributed $1.5 million to the Global Poverty Project.[44]

He has given millions of dollars to the Cambodian Children's Fund, a nonprofit program that provides a wide range of critical health and educational services to impoverished and abused children in the capital city of Phnom Penh. Redstone's contribution will be used to create the Sumner M. Redstone Child Rescue Center, a stand-alone facility scheduled to open during the fall of 2007 for children 5 to 16.[46]

In early 2010, Redstone pledged a $1 million gift to Autism Speaks in support of scientific research into the causes of autism and effective treatments. Redstone had given financial support to Autism Speaks previously.[47] In 2011, Redstone gave an additional $500,000 to the group in support of its Translational Research Initiative, bringing his cumulative lifetime contribution to Autism Speaks to $1.7 million.[48]

In September 2012, Redstone donated $18 million to the Boston University School of Law. The gift funded, in part, the construction of the five-story Sumner M. Redstone Building, a classroom building which opened in 2014.[50]

Since October 2012, Redstone has donated a total of $350,000 to the Go Campaign, which funds projects in 21 countries with a focus on helping orphans and other needy children.[51]

In May 2013, the Sumner M. Redstone Charitable Foundation donated $1 million to Literacy Inc., a New York City-based nonprofit literacy organization.[52]

In January 2014, it was announced that the Sumner M. Redstone Charitable Foundation had donated $10 million to Harvard Law School for public-interest fellowships, the largest charitable contribution ever made to the law school in support of public service. The money supports students who work in public-interest positions after graduation.[53][54]

In 1947, he married Phyllis Gloria Raphael.[55] In 1999, they divorced. They had two children: Brent Redstone and Shari Redstone. Three years after his divorce, he married Paula Fortunato, a former primary school teacher 39 years his junior. Sumner Redstone filed for divorce from her on October 17, 2008.[56] Their divorce was finalized on January 22, 2009. Redstone owns a house in the Beverly Park area of Beverly Hills, California, which he purchased in 2002 for $14,500,000.[57]

In July 2010, Redstone was caught on tape trying to find the source of an apparently embarrassing leak within his own MTV. Redstone offered money and protection to a journalist if he would give up his source. Redstone had been pushing the MTV management to give more airtime to the band the Electric Barbarellas. On the message, Redstone tells the reporter that "we're not going to kill" the source, adding "We just want to talk to him". The 87-year-old Redstone also tells the reporter he will be "well rewarded and well protected" if he would reveal the source.[58] Peter Lauria told NBC's Today show he would not do it. Viacom Inc. spokesman, Carl Folta confirmed to Today that it was Redstone's voice on the message and said the mogul had made a mistake.[59] A Viacom source told the New York Post, "Sumner wants to be consequential. Sumner is really proud of what he did. This guy is loving it… He likes people to know he's still alive".[60]

In August 2015, Redstone split with his live-in girlfriend, Sydney Holland, after five years together.[61][62]

Redstone, Sumner. "Toward More Equitable Determination of Tax Liability by Averaging of Income: An Historical and Analytical Approach to Problems of Averaging". 1947. OCLC83849763. Harvard Law School third year paper.